1. Field of the Invention
The present invention relates to a tape guide post mechanism for use in a magnetic recording/reproducing apparatus, such as a videotape recorder (VTR).
2. Description of the Related Art
In recent years, there has been a growing demand for decreases in size and cost of magnetic recording/reproduction apparatus, such as VTRs, camcorders, etc.
Hereinafter, a tape guide post mechanism for use in a conventional magnetic recording/reproducing apparatus is described.
FIG. 6 is a plan view of a magnetic recording/reproducing apparatus which uses a conventional tape guide post mechanism. FIG. 6 shows that a tape 2 has been pulled out from a cassette 1 by tape guide posts 8, 9, 10, and 13, and looped around a rotatable head cylinder 6. In this state, recording/reproduction of data or information to/from the tape 2 can be performed. In FIG. 6, some parts are omitted (i.e., not shown) for clarity of illustration.
As shown in FIG. 6, the tape 2 is wound around an S-reel 4a and a T-reel 5a stored in the cassette 1. For clarity of illustration, the external shape of the cassette 1 is represented by a dashed line. Reference numeral 3 denotes a base of a magnetic recording/reproducing apparatus, on which the cassette 1 is mounted. Reference numeral 4 denotes an S-reel bed, which is rotatably and axially supported on the base 3. The S-reel bed 4 is engaged, and rotates integrally, with the S-reel 4a. 
Reference numeral 5 denotes a T-reel bed, which is rotatably and axially supported on the base 3. The T-reel bed 5 is engaged, and rotates integrally, with the T-reel 5a. Reference numeral 15 denotes a tension arm (post arm). The tension arm 15 has: a tension post 8, which is one of the tape guide posts, and which is one of the constituent elements of a tape running system; and a hole 15a. The base 3 has a tension arm shaft 14, which is provided so as to penetrate through the hole 15a, so that the tension arm shaft 14 rotatably supports the tension arm 15. Reference numeral 18 denotes a tension band, one end of which is supported by a shaft 15b formed in the tension arm 15, and the other end which is fixed to the base 3 with a band fixture screw 19. The intermediate portion of the tension band 18 is wound around a cylindrical portion 20 of the S-reel bed 4. The tension arm 15 is urged by an urging spring 15a around the tension arm shaft 14 in a counterclockwise direction.
Reference numeral 6 denotes a rotatable head cylinder provided on the base 3. The rotatable head cylinder 6 has a rotatable magnetic head, through which a signal is recorded in/reproduced from the tape 2 that is looped around the rotatable head cylinder 6. Reference numeral 7 denotes a S1-post, which is one of the tape guide posts. Reference numeral 11 denotes a T1-post, which is one of the tape guide posts. The S1-post 7 and the T1-post 11 are fixed to the base 3, and are constituent elements of the tape running system. Reference numeral 9 denotes a S2-post, which is one of the tape guide posts. Reference numeral 10 denotes a T2-post, which is one of the tape guide posts. The S2-post 9 and the T2-post 10 are constituent elements of the tape running system. Reference numeral 12 denotes a capstan shaft, and reference numeral 100 denotes a pinch roller. The tape 2 which is sandwiched by the capstan shaft 12 and the pinch roller 100 is driven in a forward/reverse direction by forward/reverse rotation of the capstan shaft 12. Reference numeral 13 denotes a T3-post, which is one of the tape guide posts, and is a constituent element of the tape running system. The T3-post 13 is formed in a T3-arm 17 which is a post arm. The T3-arm 17 has a hole 17a. A T3-arm shaft 23a which is formed on the base 3 is provided so as to penetrate through the hole 17a, so that the T3-arm shaft 23 a rotatably supports the T3-arm 17. FIG. 6 shows that the tape 2 has been pulled out of the cassette 1 by the tension post 8, the S1-post 9, the T1-post 10, and the T3-post 13, which are driven by driving means not shown in FIG. 6.
The tape running system shown in FIG. 6 is now described. The tape 2 pulled out from the S-reel 4a is first looped around the S2-post 7 on the base 3, and then, looped around the tension post 8 and the S1-post 9. Further, the tape 2 is looped around the rotatable head cylinder 6, and around the T1-posts 10 and the T2-posts 11. Thereafter, the tape 2 is looped around the capstan shaft 12 and the T3-post 13. Thereafter, the tape 2 is wound around the T-reel 5a. 
Hereinafter, an operation of the tape guide post mechanism of the conventional magnetic recording/reproducing apparatus having the above described structure is described with reference to FIGS. 7 through 9.
FIG. 7 is a cross-sectional view showing the T3-arm 17 of the conventional tape guide post mechanism. FIG. 8 is a cross-sectional view showing the state of the T3-arm 17 when a tape tension is applied to the T3-arm 17. FIG. 9 is a perspective view showing the details about engagement between the hole 17a of the T3-arm 17 and the T3-arm shaft 23a. 
In FIG. 7, the tape 2 is not looped around the T3-post 13. The hole 17a of the T3-arm 17 is rotatably engaged with the T3-arm shaft 23 a formed on the base 3. The T3-arm 17 is prevented by a stoppage ring 21 from being upwardly disengaged from the T3-arm shaft 23a. Further, some space 22 is necessarily provided between the internal wall surface of the hole 17a and the external side surface of the T3-arm shaft 23a such that the T3-arm 17 is rotatable around the T3-arm shaft 23a. 
When the tape 2 is looped around the T3-post 13 so as to have a predetermined contact area therebetween which corresponds to a predetermined angular distance around the T3-post 13 so that the tape 2 can run, tape tension Ta is applied to the T3-post 13 as shown in FIG. 8. Due to tape tension Ta, a rotation moment is caused in the T3-arm 17 and the T3-post 13 in the direction of arrow M. Accordingly, the T3-arm 17 slightly rotates in the direction of arrow M as shown in FIG. 8. As a result, the internal wall surface of the hole 17a of the T3-arm 17 is in contact with the external side surface of the T3-arm shaft 23a at an upper contact point 30 and a lower contact point 31 as shown in FIG. 9.
FIG. 9 shows the slanted T3-arm 17 only in the vicinity of the T3-arm shaft 23a. As seen from FIG. 9, a hole centerline 33 between the center of the upper opening of the hole 17a of the T3-arm 17 (“upper hole center 32a”) and the center of the lower opening of the hole 17a of the T3-arm 17 (“lower hole center 32b”) is slanted with respect to the longitudinal axis of the T3-arm shaft 23a. Herein, the T3-post 13 is a constituent element of the tape running system. The amount and direction of slant of the T3-post 13 must be accurately determined.
However, in the above conventional structure, there are problems as described below with reference to FIGS. 8 and 9. Due to small, external disturbing factors, such as a variation of tape tension Ta, a variation of friction force generated between the T3-post 13 and the tape 2, etc., the position of the upper contact point 30 unstably shifts in a direction indicated by arrow B of FIG. 9, and the position of the lower contact point 31 unstably shifts along a direction indicated by arrow C of FIG. 9. That is, the slant of the hole center line 33 with respect to the axis of the T3-arm shaft 23a is varied, and the amount and direction of slant of the T3-post 13 are unstable. Also, shift of the positions of the upper contact point 30 and the lower contact point 31 is caused by the attitude change or vibration of the magnetic recording/reproducing apparatus, and as a result, the amount and direction of slant of the T3-post 13 are unstable.
Furthermore, the magnitude and direction of the force applied to the T3-post 13 due to the tape tension are different between a case where the tape 2 runs in a forward direction and a case where the tape 2 runs in a reverse direction. Thus, when the running direction of the tape 2 changes between the forward and reverse directions, the positions of the upper contact point 30 and the lower contact point 31 shift, so that the amount and direction of slant of the T3-post 13 are unstable. As described above, in the conventional structure, the amount and direction of slant of the T3-post 13 are essentially and necessarily unstable because of the attitude change or vibration of the magnetic recording/reproducing apparatus, a change in the running state of the tape 2, such as the running direction of the tape 2, the tape tension, or the like.